The present disclosure relates to a heat-resistant microporous film having a heat-resistant insulating layer, particularly relates to a heat-resistant microporous film and a battery separator each having a substrate and a heat-resistant layer, the substrate being made by using a polyolefin resin, and the heat-resistant layer having a structure in which inorganic particles are contained in a heat-resistant resin.
In recent years, portable electronic information apparatuses such as mobile telephones, camcorders, and notebook computers have been widely used, and such apparatuses have been therefore developed so as to have enhanced performance, reduced size, and reduced weight. These apparatuses each have a power source such as a primary battery which is disposable or a secondary battery which can be repeatedly used. In terms of good total balance in enhancement of performance, reduction in size and weight, and economic efficiency, demand for the secondary battery, particularly a lithium ion secondary battery, has been increased. In addition, in such apparatuses, the enhancement of performance and the reduction in size are further developed, and the lithium ion secondary battery is accordingly desired to also have increased energy density.
In the lithium ion secondary battery, the increase of the capacitance thereof causes energy density to be increased, and reliability in the case where large quantity of energy is released during heat emission from the battery and during the occurrence of internal short circuit is therefore significantly desired to be increased. Development of a lithium ion secondary battery which has both high reliability for such unusual situations and high capacitance is accordingly remarkably demanded.
In general, the lithium ion secondary battery has a cathode containing a lithium composite oxide, an anode containing a material which can receive and emit lithium ions, a separator disposed between the cathode and the anode, and a non-aqueous electrolyte solution. The cathode and anode are stacked with interposing the separator therebetween, or the cathode and anode are stacked and are then wound to form a column-like wound electrode. The separator has functions of electrically isolating the cathode from the anode and preserving the non-aqueous electrolyte solution. In general, a polyolefin microporous film is used as the separator of the lithium ion secondary battery having such a structure.
The polyolefin microporous film has excellent electrical insulating properties and excellent ion permeability and is therefore widely used as the separator of the lithium ion secondary battery and a capacitor. The lithium ion secondary battery exhibits high output density and high capacitance density. In contrast, in the case where unusual situations such as short circuit and overcharge occur, which cause heat emission, an organic solvent used for the non-aqueous electrolyte solution causes the non-aqueous electrolyte solution to be decomposed with the aid of the emitted heat, and fire therefore may occur in the worst cases. The lithium ion secondary battery has some safety functions to prevent such problematic phenomenon, and a shutdown function of the separator is one of such functions.
In the shutdown function of the separator, the micropores of the separator are filled with, for example, a thermally fused resin material during abnormal heat emission from the battery with the result that conduction of ions contained in the non-aqueous electrolyte solution is suppressed, and the advance of an electrochemical reaction is stopped. In general, the lower the shutdown temperature is, the higher the safety is. An appropriate shutdown temperature provided by polyethylene is one of the reasons for using polyethylene as a component of the separator. In order to form pores and increase toughness, a resin film which has been uniaxially or biaxially stretched is used as the separator.
Meanwhile, the porous separator produced in the above manner is stretched and therefore has strain. In the case where the porous separator is exposed to a high temperature environment, the porous separator problematically shrinks resulting from residual stress. The shrinkage of the separator causes secondary problems such as the occurrence of internal short circuit in some cases.
A technique is proposed in Japanese Unexamined Patent Application Publication No. 2008-123996, in which a traditional separator is provided in the form of a composite film to secure safety in a battery against the thermal shrinkage of the separator and to enhance reliability for the occurrence of internal short circuit due to various causes with the result that the problem of the shrinkage in high temperature environment is overcome.